The biological activity of selenium (Se) has been studied for over 40 years, but its precise function has not been fully elucidated. Originally identified as a highly toxic substance when ingested in large amounts, Se, usually in the form of disodium selenite (Na.sub.2 SeO.sub.3), is now recognized as an essential trace element in eukaryotes and as a potent anticarcinogenic agent in a variety of animal models (Medina, D. and D. G. Morrison (1988) Pathol. Immunopathol. Res. 7:187-199; Lane, H. W. et al. (1989) In Vivo 3:151-160; Ip, C. (1986) J. Am. Coll. Toxicol. 5:7-20; Medina D. (1986) J. Am. Coll. Toxicol. 5:7-20; Combs, G. F. and S. B Combs (1986) in The Role of Selenium in Nutrition, Academic Press, San Diego, Calif.; pp. 413-461). Dietary Se affords protection against both the initiation and promotion of carcinogensis, and there is increasing epidemiological evidence to support its anticarcinogenic role in humans (Knekt, P. et al. (1990) J. Natl. Cancer. Inst. 82:864-868).
Se-containing proteins are detected in a broad spectrum of tissues in vivo and in cell lines in vitro by labeling with trace amounts of radioactive .sup.75 Se (see for example, Morrison, D. G. et al. (1988) Anticancer Res. 8:51-64; Pederson, N. D. et al. (1972) Bioinorg. Chem. 2:33-45; Calvin, H. I. (1978) J. Exp. Zool. 204: 445-452; Motsenbocker, M. A. and A. L. Tappel (1982) Biochim. Biophys. Acta 709:160-165). Se is incorporated directly into proteins such as glutathione peroxidase, type I iodothyronine deiodinase, and selenoprotein P during translation. Incorporation of Se into these proteins is via the modified amino acid selenocysteine that is inserted into the nascent polypeptide in response to an in-frame UGA termination codon (Chambers, I. et al. (1986) EMBO J. 5:1221-1227; Takahashi, K. et al. (1990) J. Biochem. (Tokyo) 108:145-148; Berry, M. J. (1991) Nature 349:438-440; Hill, K. E. (1991) J. Biol. Chem. 266:10050-10053).
Some proteins do not contain selenocysteine but bind Se non-covalently. The precise nature of the binding site has not been established, but probably requires a pair of cysteine residues (Handel, M. L. et al. (1995) Proc. Natl. Acad. Sci. 92:4497-4501) because selenite is a potent oxidant of thiols. Additional structural elements must also play an important role in binding because only a limited number of .sup.75 Se-labeled proteins can be detected by SDS-polyacrylamide gel electrophoresis after labeling in vivo.
Several Se-binding proteins from mouse, rat, and human have been identified and characterized (Bansal M. P. et al. (1990) Carcinogenesis 11:2071-2073; Bartolone, J. B. et al. (1992) Toxicol. Appl. Pharmacol. 113:19-29; Lanfear, J. et al. (1993) Carcinogenesis 14:335-340; Handel, M. L. et al., supra; Ishii, Y. et al. (1996) Toxicol Lett. 87:1-9; Spyrou, G. (1995) FEBS Lett. 368:59-63; Chang, P. W. G. et al., unpublished). Some of them belong to a family of highly homologous cytosolic proteins with similar molecular weights (ca. 54-58 kDa) and overlapping tissue distributions in the kidney, liver, lung, gastrointestinal tract, and male and female endocrine glands.
At least two related, but separately regulated genes, for Se-binding proteins are present in the mouse (Lanfear, J. et al., supra). One of these encodes the previously described .about.58-kDa acetaminophen-binding protein (58-ABP). 58-ABP is thought to be a target for arylation by the widely used analgesic acetaminophen and its metabolites following acute drug overdose. Arylation can lead to life-threatening liver necrosis and to kidney and lung damage.
Se-binding proteins have been implicated in cellular growth control and the protection from carcinogenesis and cancer. For example, Morrison, D. G. et al. (1988; Carcinogenesis 9:1801-1810) have correlated the inhibition of DNA synthesis with the level of Se bound to proteins in mouse mammary epithelial cells in culture. Higher levels of bound Se are observed in non-growing cells than in growing cells, but the level of protein is unaffected. This suggests that Se binding modifies the activity of pre-existing proteins. Ishii, Y. et al. (supra) have also demonstrated that synthesis of an Se-binding protein is induced in rats treated with a known disease-causing polychlorinated biphenyl (PCB) compound.
Although not structurally related to the 54-58-kDa family of Se-binding proteins, the Fos and Jun subunits of the AP-1 transcription factor also bind Se and are thereby unable to bind to their DNA recognition sequence. As a consequence, transcription from AP-1-dependent promoters is inhibited by the addition of selenite to the culture media, whereas transcription from AP-2-dependent promoters is unaffected (Handel, M. et al., supra; Spyrou, G. et al., supra). The inhibition of transcription may thus provide an explanation for some of selenium's observed biological activities.
The discovery of polynucleotides encoding the selenium-binding protein, and the molecules themselves, provides the means to investigate the regulation of DNA synthesis and transcription by inorganic ions. Discovery of molecules related to the selenium-binding protein satisfies a need in the art by providing new diagnostic or therapeutic compositions useful in the diagnosis, prevention, and treatment of conditions and diseases associated with the activity of selenium-binding proteins such as liver necrosis and kidney or lung damage resulting from acetaminophen toxicity, as well as liver, kidney, lung, mammary, epithelial, gastrointestinal, and endocrine cancer.